Bryston 7B NRB-THX, ST, & SST monoblock power amplifier 7B-ST October 1996
The original 7B was intended to be Bryston's most powerful amplifier. The first designs were reverse-engineered from the company's 250Wpc 4B power amplifier by dropping that model's power-supply voltages to enable it to provide more current, then bridging the two channels (series operation) or paralleling them. The result was the Bryston 6B, a subsequent redesign resulting in the 7B. TJN reviewed the NRB-THX version of the 7B in October 1993 (Vol.16 No.10, pp.193-202), praising it for its "sweet...subtly refined...warm...full-quality" sound, which was somewhat less "lively [and] immediate" than the comparison amplifier, the now-discontinued Krell KSA-250.
As TJN's review of the 7B-NRB-THX suggested, Bryston designed the two operating modes into the 7B to enable its bridged design to handle a wide variety of loudspeaker loads. It's relatively easy if your loudspeaker has nominal impedance at either extreme—use the series configuration for a 16 ohm load (like Sound-Lab Ultimate), and use the parallel configuration if you own Infinity Kappas or Apogee Scintillas, whose impedances dip below 2 ohms. And that gray area between 3 and 8 ohms? Series configuration should be employed for loudspeakers with impedances that stay above 5 ohms across the 20Hz–20kHz frequency band; the parallel mode can be used for loudspeakers whose impedance dips below 3 ohms at any particular frequency.
The 7B-ST is similar to a bridged 3B-ST in total number of output devices. However, the 7B achieves parts savings and simpler circuitry by having only two drivers, to allow for the series mode to configure half the devices out-of-phase. When the amplifier is run in parallel mode, all output devices are tied together with a 1.5 ohm resistor. These changes give the 7B-ST much greater current-delivery capacity. The 7B-ST can be configured by back-panel slide switches in "series mode" to provide high-voltage output for speaker loads of 5 to 8 ohms, or into the high-current "parallel mode" configuration to drive loudspeakers of low (1-3 ohm) impedance. Like other bridged designs, the 7B has high power ratings, including a 500W rating for 8 ohm loads and an 800W rating for 4 ohm loads.
The 7B-ST's front panel displays the "ST Series" logo and has a rectangular power switch located at panel center. However, being a monoblock, it has only one tricolor, power-on LED. In addition, the 7B-ST is deeper and heavier than the 3B-ST, with double the heatsinking (6400cm3 of chassis vs 3200cm3) to handle the greater current draw. The 7B-ST's center chassis is 2" shallower than that of its NRB predecessor. This represents another Stuart Taylor design innovation—previously unused chassis space between the rear of the main system board and the back panel has been eliminated to give shorter wire runs between back-panel connectors and the main circuit board.
There are three toggle switches, two pairs of loudspeaker terminals, and balanced and unbalanced inputs on the back panel in addition to the detachable power cord. As on other Bryston amplifiers, white lacquer diagrams and instructions are printed on the back panel; an instruction manual is not needed. This is very important for the Bryston 7B-ST, which features unique optional bridging configurations for the speaker terminal hookups. The switches select between balanced or unbalanced inputs, series or parallel operation, and a Ground Lift switch to float the chassis ground from the signal ground.
The 7B-ST's input circuitry features the "ST" input buffer circuit, now standardized for the single-ended and balanced inputs of all ST Series amplifiers. Input circuitry is completely symmetrical. Multiple pairs of bipolar transistors are used in the output stage, each pair controlled by a single driver transistor. Output devices are hand-selected to precisely match transistor betas. Each input driver-output pair is treated as a composite output device which Brystons labels as its "Quad-Complementary" configuration. The company claims that this configuration has been fine-tuned to deliver high linearity and low-order distortion products. Open-loop distortion is claimed to be low, approximately 0.05%, and negative feedback is primarily local.
The amplifier's dual ±60V power supply uses two 500VA toroidal transformers, one for each amplifier module, again located just behind the front panel. Short lead lengths bring the 4700µF electrolytic filter caps to within 1" of the output circuitry. Internal protection circuitry is designed to handle most fault conditions, including shorts and DC-offset. A "soft-start" turn-on feature prevents a sudden huge draw on the power line when the amplifier is switched on. As TJN noted, this feature prevents you from blowing circuit breakers if your Home Theater system includes multiple pairs of 7B-STs.
Listening to the 7B-ST
The Bryston 7B-ST monoblock, the most substantial and heavy piece of electronic gear in this review, required the most care in setup. I first used a pair of 7B-STs to drive the large Snell SUB-1800 subs set up in series mode. The monoblocks were stacked, with the Ground Lift on, and driven via the unbalanced inputs. Later, I attempted to connect these amplifiers in parallel for to drive the Snell Reference Towers. This was done for two reasons. First, most of TJN's subjective impressions of the earlier 7B-NRB-THX were gathered when his amplifiers were configured in parallel mode. Second, I carried on an e-mail correspondence with Robert Deutsch, who had used the 7B-NRBs in parallel to drive various dynamic loudspeakers he'd been reviewing. There seemed to be an impression that the amplifier might sound best in the high-current parallel mode.
Switching the 7B-ST into parallel mode required bi-wired speaker cables: four leads, two per speaker terminal. My OCOS cable is bi-wired, but unfortunately, at one end, the two cables for each terminal are soldered together for convenience; this meant that I needed to run an additional set of cables. I added a pair of Levinson HFC-10s to make a total of three pairs of cables per speaker—what might be considered a "tri-wired" setup. Although this produced decent sound, I went on to disconnect (after the unit was powered down) one of the Levinson cables to make certain that it was wired to the same speaker as its companion OCOS cable. When I turned the amplifier back on, I heard a tiny pop—one of the two 7A fuses had blown. I replaced the fuse, turned the amp back on, and breathed a sigh of relief when its LED lit up green. Impressed with 7B-ST's ruggedness, I decided to conduct the remainder of the listening sessions with the 7B-STs run in series. This was the appropriate setting for all the loudspeaker loads I used for this review.
The 7B-ST made a terrific impression on first hearing: open, exciting, transparent, dynamic, effortless—and it appeared to be the optimal amplifier for the Snell Reference Towers. All the qualities I liked about the 3B-ST were there: speed, drive, slam, and superb control of the mid- and upper bass. Yet the 7B-ST had other sonic qualities not so evident in its little brother, such as its very airy, open top end, sweet midrange, and ability to render the best imaging and soundstaging I'd heard in a long time. It seemed to capture many of the best qualities of my comparison amplifiers, including the Levinson No.331's delicate detailing and transparency, and the (now discontinued) Krell KSA-250's ability to delineate the full width and depth of the soundstage. What was missing, at least in my setup, was the midrange warmth that TJN had noted in the 7B-NRB-THX.
The 7B-ST proved to be an outstanding bass amplifier—ie, from 80Hz down—when driving the huge Snell SUB-1800 subwoofers. In this configuration, the 7B-ST easily sustained the solid pedal chords of the Lay Family Concert Organ playing "The Lord is My Light and My Salvation" on the Turtle Creek Chorale's HDCD-encoded CD of the music of John Rutter (Reference Recordings RR-57CD). It equaled the Krell KSA-250 in capturing the sudden, dramatic plucked-bass and synthesizer notes during the opening of "Something's Wrong," from Randy Edelman's My Cousin Vinny soundtrack (Varèse Sarabande VSD-5). The Bryston monoblocks were lightning-fast in controlling the Snell SUB-1800's 18" subwoofer cones. I heard this best during the dramatic opening of the Eagles' "Hotel California" (from Hell Freezes Over, Geffen GEFD-24725). The band's conga section erupted with shocking power at the first hint of the melody, and played on clearly through the crowd's wild applause and foot-stomping.
The first chords of Terry Dorsey's "Ascent" on Time Warp (Telarc CD-80106) shook the room, giving ample evidence of the 7B-ST's bass "slam." This was also starkly obvious as these monoblocks delivered the burning pace generated by the tom-tom strokes and subterranean synthesizer chords on David Bowie's "Putting Out Fire," from the Cat People soundtrack (MCA 1498, LP). The 7B-ST made it as easy to discern pitch changes in the deep-bass synthesizer notes on Jeff Beck's "Behind the Veil" as did the Krell KSA-250.
The 7B-ST's bass power was particularly impressive listening to "Gnomus," from Jean Guillou's organ transcription of Mussorgsky's Pictures at an Exhibition (Dorian DOR-90117). I was most impressed with the apparent image size and sense of space around the Great Kleuker-Steinmeyer organ. There was a sense of great depth and expansiveness. The bass notes in this relatively quiet passage shuddered the air, vibrated objects in the room, and had a solidity I had not heard before. Other solid-state amplifiers on hand could not control the SUB-1800's bass response as well. The Bryston 7B-ST's speed, snap, focus, and dynamics made it my preferred amplifier for driving the Snell Reference Towers.
Switched to the Type A References' midrange-tweeter towers, the 7B-STs produced music with dynamics, power, depth of image, and airiness. All the dynamic speakers—including the Totem Model 1s, the Dahlquist DQ-10s, and the Snell Reference Towers—blossomed when driven by the Bryston 7B-STs. Snap, focus, width of soundstage, and depth of image—all were enhanced. My vinyl collection sprang to life with exciting, driving dynamics, particularly the jazz recordings. The Bryston 7B-ST depicted the placement and depth of piano and drums on Dave Grusin's rendition of "Keep Your Eye on the Sparrow," from the direct-to-disc Discovered Again LP (Sheffield Lab 5). Ron Carter's string bass and Harvey Mason's kickdrum were clearly delineated in space just behind Grusin's keyboard. The amplifier's "snap" speeded up the bass response of both dynamic systems. And the 7B-ST was able to yield enough information to discern that the bass beat was, in fact, a "floor stomp" in the opening Kyrie of Misa Criolla over the Snell Reference Towers.
Imaging from the pair of 7B-STs was first-rate, equaled only by the huge dual-mono Krell KSA-250. During the instrumental finish of Richard Thompson's "Why Must I Plead" (from Rumor and Sigh), the acoustic guitar's sonic image fell outside the right Snell Reference Tower. Soundstage depth and width were exemplary on the Holst Chaconne.
Over the past few years the Bryston 7B-ST has received accolades in the pages of Stereophile from TJN, RD, and Dick Olsher. I confirm the high quality of this monoblock amplifier; my listening revealed to me how much better the ST Series has become. The new 7B-ST has much less of the midrange and upper-bass warmth that were reported of the earlier NRB-series amplifiers, and its high power and flexible bridging arrangements are designed to handle divergent speaker loads. For example, it would be a good choice for driving high-impedance speaker loads such as the Sound-Lab Ultimate.
The Bryston 7B-ST monoblock power amplifier is an exceptional product. It's very powerful, and joins my reference Krell KSA-250 as an amplifier that can handle any loudspeaker load, play wide dynamic range music effortlessly, and excel in imaging and soundstaging. It can "wake up" reticent dynamic loudspeakers over their entire sonic range; in particular, it brought dry, reticent top ends to life. It equaled the Class A recommended amplifiers at my disposal—the Mark Levinson No.331 and the discontinued Krell KSA-250—in dynamics, transparency, and ability to transmit music's rhythmic pace.
This solid-state monoblock should be auditioned by anyone who needs a new amplifier for driving high-impedance electrostatic loudspeakers, or dynamic loudspeakers that seem somewhat bass-shy. The Bryston 7B-ST rates a solid Class A recommendation.—Larry Greenhill